556 research outputs found
Boundaries of Subcritical Coulomb Impurity Region in Gapped Graphene
The electronic energy spectrum of graphene electron subjected to a
homogeneous magnetic field in the presence of a charged Coulomb impurity is
studied analytically within two-dimensional Dirac-Weyl picture by using
variational approach. The variational scheme we used is just based on utilizing
the exact eigenstates of two-dimensional Dirac fermion in the presence of a
uniform magnetic field as a basis for determining analytical energy eigenvalues
in the presence of an attractive/repulsive charged Coulomb impurity. This
approach allows us to determine under which conditions bound state solutions
can or can not exist in gapped graphene in the presence of magnetic field. In
addition, the effects of uniform magnetic field on the boundaries of
subcritical Coulomb impurity region in the massless limit are also analyzed.
Our analytical results show that the critical impurity strength decreases with
increasing gap/mass parameter, and also that it increases with increasing
magnetic field strength. In the massless limit, we investigate that the
critical Coulomb coupling strength is independent of magnetic field, and its
upper value for the ground-state energy is 0.752.Comment: 9 pages,10 figure
An Array of Layers in Silicon Sulfides: Chain-like and Ground State Structures
While much is known about isoelectronic materials related to carbon
nanostructures, such as boron nitride layers and nanotubes, rather less is
known about equivalent silicon based materials. Following the recent discovery
of phosphorene, we herein discuss isoelectronic silicon monosulfide monolayers.
We describe a set of anisotropic ground state structures that clearly have a
high stability with respect to the near isotropic silicon monosulfide
monolayers. The source of the layer anisotropy is related to the presence of
Si-S double chains linked by some Si-Si covalent bonds, which lye at the core
of the increased stability, together with a remarkable spd hybridization on Si.
The involvement of d orbitals brings more variety to silicon-sulfide based
nanostructures that are isoelectronic to phosphorene, which could be relevant
for future applications, adding extra degrees of freedom.Comment: 16 pages, 6 figure
Colloquium: Graphene spectroscopy
Spectroscopic studies of electronic phenomena in graphene are reviewed. A
variety of methods and techniques are surveyed, from quasiparticle
spectroscopies (tunneling, photoemission) to methods probing density and
current response (infrared optics, Raman) to scanning probe nanoscopy and
ultrafast pump-probe experiments. Vast complimentary information derived from
these investigations is shown to highlight unusual properties of Dirac
quasiparticles and many-body interaction effects in the physics of graphene.Comment: 36 pages, 16 figure
Electronic, optical and transport properties of van der Waals Transition-metal Dichalcogenides Heterostructures: A First-principle Study
Two-dimensional (2D) transition-metal dichalcogenide (TMD) MX (M = Mo, W;
X= S, Se, Te) possess unique properties and novel applications. In this work,
we perform first-principles calculations on the van der Waals (vdW) stacked
MX heterostructures to investigate their electronic, optical and transport
properties systematically. We perform the so-called Anderson's rule to classify
the heterostructures by providing the scheme of the construction of energy band
diagrams for the heterostructure consisting of two semiconductor materials. For
most of the MX heterostructures, the conduction band maximum (CBM) and
valence band minimum (VBM) reside in two separate semiconductors, forming type
II band structure, thus the electron-holes pairs are spatially separated. We
also find strong interlayer coupling at point after forming MX
heterostructures, even leading to the indirect band gap. While the band
structure near point remain as the independent monolayer. The carrier
mobilities of MX heterostructures depend on three decisive factors, elastic
modulus, effective mass and deformation potential constant, which are discussed
and contrasted with those of monolayer MX, respectively.Comment: 7 figure
Probing thermal expansion of graphene and modal dispersion at low-temperature using graphene NEMS resonators
We use suspended graphene electromechanical resonators to study the variation
of resonant frequency as a function of temperature. Measuring the change in
frequency resulting from a change in tension, from 300 K to 30 K, allows us to
extract information about the thermal expansion of monolayer graphene as a
function of temperature, which is critical for strain engineering applications.
We find that thermal expansion of graphene is negative for all temperatures
between 300K and 30K. We also study the dispersion, the variation of resonant
frequency with DC gate voltage, of the electromechanical modes and find
considerable tunability of resonant frequency, desirable for applications like
mass sensing and RF signal processing at room temperature. With lowering of
temperature, we find that the positively dispersing electromechanical modes
evolve to negatively dispersing ones. We quantitatively explain this crossover
and discuss optimal electromechanical properties that are desirable for
temperature compensated sensors.Comment: For supplementary information and high resolution figures please go
to http://www.tifr.res.in/~deshmukh/publication.htm
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